It is well known that a mixture of a gaseous fuel such as a compressed natural gas and an air is combusted in a combustion chamber of an internal combustion engine and a piston of the internal combustion engine reciprocates according a combustion power of the mixture. A natural gas can have a stable ignitionability by being uniformly mixed with the air. According to JP-2007-278215A, in a gaseous fuel feed apparatus, plural fuel injection ports are provided at different positions of an intake manifold in an axial direction of the intake manifold, so as to advance a mixing of the gaseous fuel and the air.
However, in the gaseous fuel feed apparatus according to JP-2007-278215A, it is necessary to increase a total number of the fuel injection ports to improve an effect of the above configuration. Therefore, the intake manifold may become longer, and a responsivity of the internal combustion engine may be deteriorated.
Further, since the natural gas has a specific gravity less than a specific gravity of the air, the natural gas is readily gathered in an upper area of the intake passage. Thus, even though the gaseous fuel is injected from different positions of the intake manifold in the axial direction of the intake manifold, the natural gas is gathered in an upper area of the intake passage corresponding to the different positions, and it is difficult to determine whether the mixing of the gaseous fuel and the air is advanced.